Color separation apparatus, color separation method, and non-transitory computer readable medium

ABSTRACT

A color separation apparatus comprising: a target value acquisition device; a dot threshold data acquisition device which acquires dot threshold data including information on a threshold for each of the dots for converting the continuous-tone image data into binary image data for each of the color materials; a print profile acquisition device which acquires a print profile showing correspondence between a device signal value and a value of a color system in the printer; and a color separation device which allows the printer to calculate candidates of the device signal value on the basis of the target values of colors acquired by the target value acquisition device and the print profile, and determines a device signal value for reproducing colors corresponding to the target values from among the candidates of the device signal value on the basis of the dot threshold data and the print profile.

CROSS-REFERENCE TO RELATED APPLICATIONS

The patent application claims priority under 35 U.S.C. §119 to JapanesePatent Application No. 2013-180158, filed on Aug. 30, 2013. Each of theabove application(s) is hereby expressly incorporated by reference, inits entirety, into the present application.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a color separation apparatus, a colorseparation method, and a color separation program, and more particularlyto a color separation apparatus, a color separation method, and a colorseparation program, for applying color separation to image data forprinting to make colors to be used in a printer.

2. Description of the Related Art

When image data is printed, color separation is applied to the imagedata to make a dot image for each of color materials (process colors(CMYK), for example), and overprinting is performed by using a printingplate on which dots corresponding to each of the color materials areformed so that gradation of the image data is expressed. In a case wheredot images corresponding to a plurality of color materials areoverprinted, a pattern with an image formation such as a moire and arosette pattern sometimes occurs depending on a position and an angle ofa dot, screen ruling, and a shape.

Japanese Patent Application Laid-Open No. 2007-116287 (patentliterature 1) discloses a method of evaluating noise information havingstrong periodicity in dot images when color separation is applied toimage data. In Japanese Patent Application Laid-Open No. 2007-116287, aprinted patch image is read, and each pixel data of the image data isresolved into RGB data to be transferred to an optimization unit 12 bybeing converted into color information (L*, a*, and b*, for example) ina uniform color space. The optimization unit 12 calculates a graininessevaluation value on the basis of L* based on parameter of granularityand ink concentration determined by a UI unit 11 (refer to theparagraphs [0019] to [0023]).

SUMMARY OF THE INVENTION

According to Japanese Patent Application Laid-Open No. 2007-116287, whencolor separation is performed, patches corresponding to all combinationsof device signal values (256 colors per one color) are printed so thatcolor information is obtained from an image of the patch, whereby anevaluation value of granularity is calculated. Thus, when colorseparation into two colors is performed, it is necessary to print alarge number of patches of 256×256 (=65536) for measurement of thepatches. Accordingly, when color separation into four colors of CMYK isperformed, it takes time because an enormous number of patches areprinted for measurement of the patches. Further, since an evaluationvalue such as granularity varies depending on a type of a dot, operationof printing and measurement is required for each type of a dot to causea large load of the operation.

The present invention is made in light of the above-mentionedcircumstances, and an object of the present invention is to provide acolor separation apparatus, a color separation method, and a colorseparation program, capable of easily performing color separation forpreventing an image formation such as a moire and a rosette pattern fromoccurring.

In order to solve the problem described above, a color separationapparatus according to a first aspect of the present invention includes:a target value acquisition device which acquires target values of colorsto be reproduced in a printer from continuous-tone image data when theprinter creates binary image data showing shape and arrangement of dotsconstituting an image for each color materials; a dot threshold dataacquisition device which acquires dot threshold data includinginformation on a threshold for each of the dots for converting thecontinuous-tone image data into binary image data for each of the colormaterials; a print profile acquisition device which acquires a printprofile showing correspondence between a device signal value and a valueof a color system in the printer; and a color separation device whichallows the printer to calculate candidates of the device signal value onthe basis of the target values of colors acquired by the target valueacquisition device and the print profile, and determines a device signalvalue for reproducing colors corresponding to the target values fromamong the candidates of the device signal value on the basis of the dotthreshold data and the print profile.

According to the present aspect, it is possible to achieve optimum colorseparation capable of preventing a color difference and preventing animage formation from occurring by a simpler calculation without printingand measuring a patch for each device signal value.

A color separation apparatus according to a second aspect of the presentinvention is the color separation apparatus according to the firstaspect, further including a color evaluation value holding device whichholds a color evaluation value showing a color difference between adevice signal value and a color corresponding to a target value for eachof the candidates of the device signal values; and an image qualityevaluation value calculation device which calculates an image qualityevaluation value for each of the candidates of the device signal valueson the basis of the dot threshold data and the print profile, and in thesecond aspect the color separation device is configured to determinedevice signal values for reproducing colors corresponding to targetvalues on the basis of the color evaluation value and the image qualityevaluation value.

A color separation apparatus according to a third aspect of the presentinvention is the color separation apparatus according to the secondaspect, further including a simulation device which applies binary codedprocessing to the candidates of the device signal values by using thedot threshold data to create binary image data for each of colormaterials for each of the candidates, and simulates an image formed bythe printer on a printed matter by superimposing binary image data foreach of the color materials, and in the third aspect, the image qualityevaluation value calculation device is configured to calculate the imagequality evaluation value for each of the candidates on the basis of aresult of the simulation of the image to be formed on the printedmatter.

A color separation apparatus according to a fourth aspect of the presentinvention is the color separation apparatus according to the thirdaspect, in which the simulation device creates the binary image data inconsideration of change in a shape of the dot occurring when the dot isprinted by the printer on the basis of response characteristics at thetime of forming dots corresponding to the binary image data in theprinter.

A color separation apparatus according to a fifth aspect of the presentinvention is the color separation apparatus according to one of thesecond to fourth aspects, further including an ink cost evaluation valuecalculation device which calculates an ink cost evaluation value foreach of the candidates of the device signal values on the basis of a dotarea rate of each of color materials, and in the fifth aspect, the colorseparation device is configured to determine the device signal valuesfor reproducing colors corresponding to target values on the basis of acolor evaluation value, an image quality evaluation value, and an inkcost evaluation value.

A color separation apparatus according to a sixth aspect of the presentinvention is the color separation apparatus according to one of thesecond to fifth aspects, further including a weight coefficient settingdevice which determines a weight coefficient for each of evaluationvalues; and a total evaluation value calculation device which calculatesa total evaluation value for each of the candidates of the device signalvalues by performing weighting addition of each of the evaluation valuesby using the weight coefficient determined by the weight coefficientsetting device, and in the sixth aspect, the color separation device isconfigured to determine device signal values for reproducing colorscorresponding to target values on the basis of the total evaluationvalue.

A color separation apparatus according to a seventh aspect of thepresent invention is the color separation apparatus according to one ofthe first to sixth aspects, in which the target value acquisition deviceacquires target values of colors to be reproduced in a printer byreceiving input of an identifier for identifying a color or by measuringthe colors.

An eighth aspect of the present invention is a color separation methodperformed by a color separation apparatus and includes: a target valueacquisition step of acquiring target values of colors to be reproducedin a printer from continuous-tone image data when the printer createsbinary image data showing shape and arrangement of dots constituting animage for each color material; a dot threshold data acquisition step ofacquiring dot threshold data including information on a threshold foreach dot for converting the continuous-tone image data into binary imagedata for each color material; a print profile acquisition step ofacquiring a print profile showing correspondence between a device signalvalue and a value of a color system in the printer; and a colorseparation step of allowing the printer to calculate candidates of thedevice signal value on the basis of the target values of colors acquiredin the target value acquisition step and the print profile, anddetermines a device signal value for reproducing colors corresponding tothe target values from among the candidates of the device signal valueon the basis of the dot threshold data and the print profile.

A non-transitory computer-readable medium storing a color separationprogram according to a ninth aspect of the present invention allowing acomputer to realize the functions of: a target value acquisition ofacquiring target values of colors to be reproduced in a printer fromcontinuous-tone image data when the printer creates binary image datashowing shape and arrangement of dots constituting an image for each ofcolor materials; a dot threshold data acquisition of acquiring dotthreshold data including information on a threshold for each dot forconverting the continuous-tone image data into binary image data; aprint profile acquisition of acquiring a print profile showingcorrespondence between a device signal value and a value of a colorsystem in the printer; and a color separation of allowing the printer tocalculate candidates of the device signal value on the basis of thetarget values of colors acquired by the target value acquisitionfunction and the print profile, and determining a device signal valuefor reproducing colors corresponding to the target values from among thecandidates of the device signal value on the basis of the dot thresholddata and the print profile.

According to the present invention, since color separation is performedby using an image quality evaluation value calculated by using a powerspectrum of a dot overlapping solid brightness image and a powerspectrum of visual characteristics in addition to a color differenceevaluation value showing an amplitude of a color difference, it ispossible to prevent an image formation such as a moire and a rosettepattern from occurring while a color difference occurring between a spotcolor and a color system is prevented. Further, by performing dotoverlapping simulation, it is possible to achieve optimum colorseparation capable of preventing a color difference and preventing animage formation from occurring by a simpler calculation without printingand measuring a patch for each device signal value.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing a configuration of an image formationsystem in accordance with one embodiment of the present invention;

FIG. 2 is a block diagram showing a configuration of an image processingapparatus in accordance with one embodiment of the present invention;

FIG. 3 is a diagram to explain basic processing of dot overlappingsimulation;

FIG. 4 is a diagram to explain processing of creating a dot image with asingle color from candidates of device signal values (CMYK);

FIG. 5 is a block diagram showing a configuration of a dot overlappingsimulation processing unit when processing of estimating a dot shapechange is performed;

FIG. 6 is a block diagram showing a configuration of an on-plate dotshape estimation processing unit;

FIG. 7 is a block diagram showing a configuration of anon-printed-matter dot shape estimation processing unit;

FIG. 8 is a block diagram to explain processing of estimating systemresponse characteristics;

FIG. 9A is a plan view showing dot image data for acquiring responsecharacteristics, FIG. 9B is a plan view showing dot shapes on a printingplate (actual plate), and FIG. 9C is a plan view showing dot shapes on aprinted matter (actual printed matter);

FIGS. 10A to 10F are graphs showing examples of exposure responsecharacteristics;

FIG. 11 is a block diagram showing a configuration of an exposureresponse characteristics estimation unit;

FIG. 12 is a block diagram showing a configuration of a printingresponse characteristics estimation unit;

FIG. 13 is a block diagram showing calculation processing in an imagequality evaluation value calculation unit;

FIG. 14 is a graph showing visual characteristics (Dooley-Shaw function)in a case where an observation distance (L) is set at 300 mm;

FIGS. 15A to 15D are graphs showing examples of a power spectrum of adot overlapping solid brightness image;

FIG. 16 is a flow chart showing color separation processing according toone embodiment of the present invention; and

FIG. 17 is a diagram showing a GUI to select a color separation result.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to accompanying drawings, embodiments of the color separationapparatus, the color separation method, and the color separationprogram, in accordance with the present invention, will be describedbelow.

A configuration of an image formation system will be described below.

FIG. 1 is a block diagram showing a configuration of an image formationsystem in accordance with one embodiment of the present invention.

As shown in FIG. 1, an image formation system 10 in accordance with thepresent embodiment includes an image processing apparatus 100,plate-making device 200, and a printer 300.

Printing image data D10 includes a continuous-tone image signal. Theimage processing apparatus 100 applies color separation processing tothe printing image data D10 received to create plate-making data D20showing a dot image for each of color materials (in the presentembodiment, four colors of cyan (C), magenta (M), yellow (Y), and black(K)) used in the printer 300, and outputs the plate-making data D20 tothe plate-making device 200.

The plate-making device 200 serves as a Computer To Plate (CTP) drawingdevice which creates a printing plate 250 provided with protrusionscorresponding to a plurality of dots on the basis of the plate-makingdata D20 supplied from the image processing apparatus 100. Theplate-making device 200 is provided with a laser engraving machine, andcreates the printing plate 250 by engraving the protrusionscorresponding to a plurality of dots on a principal surface of theprinting plate 250. In a case of flexographic printing, a high elasticmaterial such as a rubber sheet and a photocurable resin sheet is usedas a material of the printing plate 250, for example.

The printer 300 applies a color material (ink) to the printing plate 250created by the plate-making device 200 and allows the ink to transfer toa printing surface of a recording medium (recording paper) to form aprinted matter 400. In a case of the flexographic printing, a printingmedium with rough surface (such as corrugated cardboard, film, andcloth) is used as the recording paper in addition to sheet paper androll paper for printing. The printer 300 as well as the plate-makingdevice 200 constitutes an image forming apparatus which forms an imageon a printing surface of recording paper.

A configuration of an image processing apparatus will be describedbelow.

FIG. 2 is a block diagram showing a configuration of an image processingapparatus (color separation apparatus) in accordance with one embodimentof the present invention.

The image processing apparatus 100 shown in FIG. 2 acquires targetvalues of colors to be reproduced in the printer 300. In addition, theimage processing apparatus 100 performs color separation processing toacquire an optimum device signal value on the basis of the acquiredtarget values of colors above and a print profile in the printer 300.

FIG. 2 shows the following: a spot color data base (DB) 102 and acolorimetry unit 104, which constitute a target value acquisitiondevice; a dot threshold data storage unit 112 and a dot overlappingsimulation unit 114, which constitute a dot threshold data acquisitiondevice; a device signal value candidate calculation unit 106 and a printprofile storage unit 108, which constitute a print profile acquisitiondevice; and the device signal value candidate calculation unit 106, acolor evaluation value holding unit 110, a dot overlapping simulationunit 114, an image quality evaluation value calculation unit 116, atotal evaluation value calculation unit 118, and an optimal valuesdetermination unit 120, which constitute a color separation device.

In the present embodiment, an example, in which the L* a* b* colorsystem (established by Japanese Industrial Standards Committee (JIS) Z8729) is used as a color system showing target values of colors and acolor system in a print profile, is described, however, a color systemof the present invention is not limited to the color system above. It ispossible to use the following: the XYZ color system (a stimulus value Yincluding luminance (brightness), and stimulus values of color X and Z)established by Commission International e de l'Eclairage (CIE): the Yxycolor system (luminance Y, and chromaticity coordinates x and y); the L*u* v* color system (established by JIS Z 8518); as well as the HSV colorsystem (hue H (hue), saturation S (saturation), and brightness V (value)or B (brightness)); the HLS color system (hue H (hue), saturation S(saturation), luminance L (luminance)); and the YCbCr color system(luminance Y, color differences Cb and Cr), for example.

Acquisition of spot colors will be described below.

The spot color data base (spot color DB) 102 stores a look-up table(LUT) showing correspondence between a number (identifier and spot colorID) of a spot color in a color sample book of an ink manufacturer, suchas PANTONE, DIC, and TOYO, and color system values (in the presentembodiment, Lab values (brightness value L*, chromaticity values a* andb*)) corresponding to a spot color ID. The image processing apparatus100 receives input of a spot color ID to identify a color to be atarget, which is to be reproduced on the printed matter 400, from auser, and acquires color system values (Lab values) corresponding to thereceived spot color ID from the spot color DB 102.

The colorimetry unit 104 is provided with a colorimetry machine (anoptical colorimetry machine such as a spectrophotometric colorimeter,for example). The colorimetry unit 104 measures color system values (Labvalues) corresponding to a spot color sample (a color chip and a printedmatter, for example) to which a spot color to identify the target colorto be reproduced on the printed matter 400 is applied, or a color of anobject whose color can be measured (an object to be a color sample) byusing a colorimetry machine.

An Lab value of the spot color identified by the spot color ID describedabove, or an Lab value (a target value showing the target color to bereproduced on the printed matter 400) of the spot color measured by thecolorimetry machine is outputted to the device signal value candidatecalculation unit 106.

In the present embodiment, an example, in which color separation intofour colors of CMYK is applied, is described, however, the number of anda type of the color materials in the printer 300 are not limited to thefour colors above. The color separation method in accordance with thepresent invention is also applicable to a case of three color materials(CMY, for example), for example. In addition, the color separationmethod in accordance with the present invention is also applicable to acase of color materials of five or more, such as: five colors ofCMYK+one additional spot color (a color specified by a user, or a colorselected by the image processing apparatus 100 on the basis of hue of acolor specified by a user, for example); six colors of CMYK+additionaltwo spot colors (“orange (O) and green (G)”, or “orange (O) and violet(V)”, for example); and seven colors of CMYK+additional three spotcolors (for example, “red (R), green (G), and blue (B)”, or “O, G, andV”).

Calculation of a device signal value candidate and a color evaluationvalue will be described below.

The print profile storage unit 108 stores a look-up table (LUT; a printprofile to convert device signal values into color system values (Labvalues)) showing correspondence between device signal valuescorresponding to color materials (process colors (CMYK), for example)and color system values (in the present embodiment, Lab values). It ispossible to use an A2B1 table of a profile of the International ColorConsortium (ICC) as the print profile, for example. There are an A2B0table (Perceptual (perceptual coincidence)) and an A2B2 table(Saturation (saturation is emphasized)) serving as a method of colorspace conversion other than the A2B1 table (Relative Colormetric(colorimetry coincidence)). If the A2B1 table is adopted, a relativecolor gamut is maintained, therefore, it is possible to minimizeinfluence of color gamut conversion (compression).

The device signal value candidate calculation unit 106 acquires a printprofile for converting device signal values corresponding to colormaterials (process colors (CMYK), for example) into color system values(Lab values) from the print profile storage unit 108. In addition, thedevice signal value candidate calculation unit 106 performs inverseoperation by using the print profile to calculate candidates ((c1, m1,y1, k1), (c2, m2, y2, k2), (c3, m3, y3, k3), . . . ) of device signalvalues (CMYK values in the printer 300), which enable target Lab values(target Lab values), which is to be reproduced on recording paper, to bereproduced within a range of an allowable color difference.

Specifically, the device signal value candidate calculation unit 106searches for CMYK values satisfying conditions of Expression 1 below,where an allowable color difference is indicated as dEa, target Labvalues is indicated as (Lt, at, bt), and output values (Lab values) when(C, M, Y, K) is inputted into a print profile (LUT) are indicated as(Lp, ap, bp)=LUT (C, M, Y, K), to acquire candidates of the CMYK values.

$\begin{matrix}{{{dE}\left( {C,M,Y,K} \right)} = {\sqrt{\left( {{Lt} - {Lp}} \right)^{2} + \left( {{at} - {ap}} \right)^{2} + \left( {{bt} - {bp}} \right)^{2}} \leq {dEa}}} & \left\lbrack {{Expression}\mspace{14mu} 1} \right\rbrack\end{matrix}$

In Expression 1, a user may specify a value of the allowable colordifference dEa in consideration of color reproducibility and the like inthe printer 300. In addition, the value of the allowable colordifference dEa may be predetermined (3, for example).

It is possible to adopt the Newton method and the like as the method ofsearching for candidates of CMYK values. In the search for thecandidates of the CMYK values, it is allowed to search for all CMYKvalues satisfying the conditions of Expression 1.

In addition, at the time of calculating candidates of device signalvalues, an allowable color difference may not be used. In this case, atotal number (N) of candidates of device signal values is predeterminedso that N candidates in ascending order of a color difference dE in asearch range may be selected, for example.

The color difference dE corresponding to each of the candidates of theCMYK values, which are acquired as the result of the search, isoutputted from the device signal value candidate calculation unit 106 tothe color evaluation value holding unit (color evaluation value holdingdevice) 110. The color evaluation value holding unit 110 holds the colordifference dE as a color evaluation value (E) corresponding to each ofthe candidates of the CMYK values.

Expression 1 above serves as a calculation expression of a colordifference ΔE*ab in an L* a* b* color system, however, anothercalculation expression of a color difference (the calculation expressionof a color difference ΔE00 in the Commission International d'Eclairage2000 (CIE DE 2000), for example) may be used. In addition, a type of acalculation expression of a color difference to be used in search forcandidates of the CMYK values can be specified by a user.

In addition, even in a case where a color system other than the L* a* b*color system is used, it is possible to calculate a color difference bya method similar to the method described above.

Dot overlapping simulation processing will be described below.

The dot threshold data storage unit 112 stores dot threshold data tobinarize continuous-tone image data for each of color materials (processcolors (CMYK)).

The dot overlapping simulation unit (simulation device) 114 estimates(simulates) an image to be formed when binary image data correspondingto each of color materials (CMYK) is printed. Specifically, the dotoverlapping simulation unit 114 creates simple (tint) dot image data(binary image data) for each of the color materials (CMYK), from each ofthe candidates of the device signal values calculated by the devicesignal value candidate calculation unit 106 by using dot threshold datadescribed above.

FIG. 3 is a diagram to explain basic processing of dot overlappingsimulation. FIG. 4 is a diagram to explain processing of creating a dotimage with a single color from candidates of device signal values(CMYK).

First, the dot overlapping simulation unit 114 acquires dot thresholddata corresponding to each of color materials (process colors (CMYK))from the dot threshold data storage unit 112. As shown in FIG. 3, thenthe dot overlapping simulation unit 114 applies raster conversion to thecandidates of the device signal values (continuous-tone image data foreach of color materials (CMYK)) calculated by the device signal valuecandidate calculation unit 106 to acquire raster data corresponding tothe candidates of the device signal values. The dot overlappingsimulation unit 114 then compares the raster data acquired from thecandidates of the device signal values and the dot threshold data tocreate (tint) dot image data (binary image data) for each of the colormaterials (CMYK).

Specifically, as shown in FIG. 4, in a case where a pixel value (G) ofthe device signals calculated by the device signal value candidatecalculation unit 106 is more than a value (T) of a corresponding pixelof the dot threshold data (G>T), the dot overlapping simulation unit 114indicates the value of the pixel in binary image data as “1”. On theother hand, in a case where a pixel value of the device signals is equalto or less than a value of a corresponding pixel of the dot thresholddata (G≦T), the dot overlapping simulation unit 114 indicates the valueof the pixel in binary image data as “0”.

Next, as shown in FIG. 3, the dot overlapping simulation unit 114creates a dot overlapping solid brightness image including informationon a brightness value (L*) of each of pixels by superimposing a binaryimage for each of the color materials (CMYK).

The dot overlapping solid brightness image serves as an image withsixteen brightness values (L* values) corresponding to the number ofcombinations of values (binary of a dot area rate of 0% or 100%) of theCMYK superimposed with each other. It is possible to acquire brightnessvalues (L* values) with respect to sixteen of the number of thecombination of the CMYK from a print profile. The dot overlapping solidbrightness image created as above is outputted to the image qualityevaluation value calculation unit 116.

Table 1 shows an example of brightness values (L* values) acquired froma print profile. The brightness values (L* values) in Table 1 may varydepending on a type of the printer 300.

TABLE 1 Solid C (%) M (%) Y (%) K (%) L* value W (Paper White) 0 0 0 0100.0 C 100 0 0 0 56.5 M 0 100 0 0 49.3 Y 0 0 100 0 93.9 K 0 0 0 10012.8 CM 100 100 0 0 18.4 CY 100 0 100 0 49.7 CK 100 0 0 100 7.7 MY 0 100100 0 47.8 MK 0 100 0 100 7.6 YK 0 0 100 100 13.2 CMY 100 100 100 0 14.9CMK 100 100 0 100 4.9 CYK 100 0 100 100 7.4 MYK 0 100 100 100 7.8 CMYK100 100 100 100 5.0

In the present embodiment, dot overlapping simulation is performed byusing a brightness value L* of an L* a* b* color system, however, thepresent invention is not limited to the brightness value. In a casewhere the XYZ color system, the Yxy color system, the YCbCr colorsystem, or the HLS color system, is used, for example, the dotoverlapping simulation may be performed by using a luminance imageshowing distribution of luminance values (Y or L).

In addition, the dot overlapping simulation may be performed by using animage showing distribution of a component (chromaticity (chromaticityvalues a* and b* of the L* a* b* color system) or saturation (saturationS of the HSV color system and the HLS color system), for example) otherthan brightness values or luminance values. In this case, it is possibleto evaluate a noise and an image formation caused by a color tone.

Further, the dot overlapping simulation may be performed by usingbrightness values or luminance values, as well as a component other thanthem. In this case, it is possible to evaluate not only a noise and animage formation (a moire and a rosette pattern, for example) caused bybrightness but also a noise and an image formation caused by a colortone.

Dot overlapping simulation processing including processing of estimatingchange in a dot shape will be described below.

In a case where dot overlapping simulation is performed by assuming aprinting system (off-set printing, etc.) with a relatively smalldifference between dots in device signal value data and dots on theprinted matter 400, it is sufficient to perform basic processing shownin FIG. 3. On the other hand, in a case where dot overlapping simulationis performed by assuming a printing system (flexographic printing, etc.)with a relatively large difference between dots in device signal valuedata and dots on the printed matter 400, it is preferable thatprocessing of estimating change in a dot shape (dot spreadingsimulation) is added to steps before a single color (tint) dot image isacquired in the basic processing, as shown in FIG. 5.

FIG. 5 is a block diagram showing a configuration of a dot overlappingsimulation processing unit when processing of estimating dot shapechange is performed.

In an example shown in FIG. 5, the dot overlapping simulation unit 114includes a binarization processing unit 150, an on-plate dot shapeestimation unit 152, and an on-printed-matter dot shape estimation unit154. In addition, in the example shown in FIG. 5, two steps of on-platedot shape estimation processing and on-printed-matter dot shapeestimation processing are performed between processing of thebinarization processing unit 150 and creation processing of single color(tint) dot image data.

System response characteristics data can vary depending on aconfiguration of the image formation system 10, and stored in a memoryof the dot overlapping simulation unit 114, for example. The systemresponse characteristics data includes exposure (engraving) responsecharacteristics data and printing response characteristics data(transfer response characteristics data).

The exposure response characteristics data shows optical characteristicsof a laser engraving machine provided in the plate-making device 200,and is defined as a Point Spread Function (PSF) showing an engravablerange (a position and a size) on the printing plate 250 when protrusionscorresponding to dots are engraved (exposed) on the printing plate 250.

The printing response characteristics data shows transfercharacteristics when ink is transferred to recording paper in theprinter 300, and is defined as a PSF showing a range (a position and asize) of points to be reproduced on the printed matter 400 when dots areprinted by using the protrusions formed on the printing plate 250.

The on-plate dot shape estimation unit 152 estimates a differencebetween a dot shape in data and a dot shape to be formed on the printingplate 250 when dots are formed on the printing plate 250 by theplate-making device 200 on the basis of exposure (engraving) responsecharacteristics data.

The on-printed-matter dot shape estimation unit 154 estimates adifference between a shape of a dot formed on the printing plate 250 bythe plate-making device 200 and a shape of a dot to be formed on theprinted matter 400 by using the printing plate 250 on the basis of theprinting response characteristics data (transfer responsecharacteristics data).

As shown in FIG. 5, the system response characteristics data serves asdata showing change in a dot shape occurring in steps until an image iscreated on recording paper after a binary image signal is created by thebinarization processing unit 150. The system response characteristicsdata varies depending on a configuration of the image formation system10.

FIG. 6 is a block diagram showing a configuration of the on-plate dotshape estimation unit 152, and FIG. 7 is a block diagram showing aconfiguration of an on-printed-matter dot shape estimation unit 154.

The on-plate dot shape estimation unit 152, as shown in FIG. 6, appliesFFT processing to each of the dot image data received from thebinarization processing unit 150 and the exposure responsecharacteristics data to perform multiplication of them. The on-plate dotshape estimation unit 152 then applies high speed Fourier inversetransformation (iFFT) processing to the data acquired by themultiplication to create on-plate dot image data showing arrangement andshapes of dots on the printing plate 250, and outputs the data to theon-printed-matter dot shape estimation unit 154.

The on-printed-matter dot shape estimation unit 154, as shown in FIG. 7,applies FFT processing to each of the on-plate dot image data receivedfrom the on-plate dot shape estimation unit 152 and the printingresponse characteristics data to perform multiplication of them. Theon-printed-matter dot shape estimation unit 154 then applies high speedFourier inverse transformation (iFFT) processing to the data acquired bythe multiplication to create on-printed-matter dot image data showingarrangement and shapes of dots on the printed matter 400.

The dot overlapping simulation unit 114 creates (tint) dot image data(binary image) for each colors of CMYK on the basis of theon-printed-matter dot image data.

In the example shown in FIG. 6 and FIG. 7, dot shape estimationprocessing is performed by multiplication in a Fourier space, however,the dot shape estimation processing may be performed by convolutionoperation in a real space.

Estimation processing of system response characteristics will bedescribed below.

If system response characteristics data is not provided in advance, thesystem response characteristics can be estimated as described below.

FIG. 8 is a block diagram to explain processing of estimating systemresponse characteristics, and FIGS. 9A, 9B, and 9C, are plan viewsshowing dot image data for response characteristics acquisition, dotshapes on a printing plate (real plate), and dot shapes on a printedmatter (real printed matter), respectively.

As shown in FIG. 8, an estimation system of system responsecharacteristics in accordance with the present embodiment includes: aplate measurement unit 500; a printed matter measurement unit 502; anexposure response characteristics estimation unit 504; and a printingresponse characteristics estimation unit 506. A measurement value of anon-plate dot area rate measured by the plate measurement unit 500 and ameasurement value of an on-printed-matter dot area rate measured by theprinted matter measurement unit 502 can be inputted into the exposureresponse characteristics estimation unit 504 and the printing responsecharacteristics estimation unit 506, respectively, through apredetermined communication device or an input device.

In an example of FIG. 8, the plate-making device 200 receives input ofthe dot image data for response characteristics acquisition to createthe printing plate (real plate) 250A. The printer 300 creates theprinted matter (real printed matter) 400A by using the printing plate250A.

The plate measurement unit 500 serves as a device for measuring shapesof dots on the printing plate (real plate) 250A created by theplate-making device 200 and an on-plate dot area rate showing a ratio ofarea occupied by dots (a region A12 in FIG. 9B) in a region (a regionA10 in FIG. 9B) of a predetermined area (unit area) on the printingplate 250A. The measurement value of the on-plate dot area rate isinputted from the plate measurement unit 500 to the exposure responsecharacteristics estimation unit 504. In addition, the vipFLEX of X-Ritemake(http://www.sdg-net.co.jp/products/x-rite/products_detail/vip_flex.html),the FlexoCam of Provident make (http://www.providentgrp.com/), and thelike are available as the plate measurement unit 500, for example.

The printed matter measurement unit 502 measure shapes of dots on theprinted matter (real printed matter) 400A created by using the printingplate 250A and an on-printed-matter dot area rate showing a ratio ofarea occupied by dots in a region of a predetermined area (unit area) onthe printed matter 400A.

The measurement value of the on-printed-matter dot area rate is inputtedfrom the printed matter measurement unit 502 to the printing responsecharacteristics estimation unit 506. In addition, the SpectroPlate ofTechkon make (http://www.techkon.co.jp/Products_Techkon_SP_Top.html),the 500 series of X-Rite make(http://www.sdg-net.co.jp/products/x-rite/products_detail/500_series.html),and the like are available as the printed matter measurement unit, forexample.

The exposure response characteristics estimation unit 504 and theprinting response characteristics estimation unit 506 may be provided inan apparatus separate from the image processing apparatus 100 or in theimage processing apparatus 100. In a case where the exposure responsecharacteristics estimation unit 504 and the printing responsecharacteristics estimation unit 506 are provided in the image processingapparatus 100, it is possible to input acquisition results by theexposure response characteristics estimation unit 504 and the printingresponse characteristics estimation unit 506 into the image processingapparatus 100 by communicatively connecting the exposure responsecharacteristics estimation unit 504 and the printing responsecharacteristics estimation unit 506 to the image processing apparatus100.

Estimation processing of exposure (engraving) response characteristicswill be described below.

FIGS. 10A to 10F are graphs showing examples of exposure responsecharacteristics, and FIG. 11 is a block diagram showing a configurationof the exposure response characteristics estimation unit 504.

As shown in FIGS. 10A to 10C, when dots are exposed (engraved) on amaterial (plate material) of the printing plate 250A, exposure(engraving) response characteristics showing an engravable range on asurface of the plate material is supposed as a rectangular function(delta function) (in fact, the exposure response characteristics issupposed as a cylindrical function on a two-dimensional plane (platematerial surface, or xy plane) as shown in FIGS. 10D to 10F, however, itis supposed as above for easy explanation). The exposure responsecharacteristics indicate spread of portions other than dots (a whiteregion), which occurs when portions other than dots (a white region inFIG. 9B) are exposed with a laser at the time of exposure (engraving). Aconcept of the exposure response characteristics is similar to that of a“Point Spread Function” (PSF) indicating spread of a point light sourcein an optics field and the like.

Next, a calculation value of an on-plate dot area rate corresponding toa dot image for characteristics acquisition is calculated by usingtemporary exposure response characteristics acquired by varying a widthw (radius r of the circle of a cylindrical function in a case of atwo-dimensional plane) of a rectangular function. Specifically, as shownin FIG. 11, the exposure response characteristics estimation unit 504receives input of dot image data for response characteristicsacquisition and temporary exposure response characteristics to apply FFTprocessing to the dot image data for response characteristicsacquisition and the temporary exposure response characteristics. Theexposure response characteristics estimation unit 504 then multipliesthe dot image data for response characteristics acquisition and thetemporary exposure response characteristics together after the FFTprocessing is applied to them, and then creates an on-plate dot imageshowing arrangement and shapes of dots to be formed on the printingplate 250A on the basis of the temporary exposure responsecharacteristics by applying iFFT processing to them. The exposureresponse characteristics estimation unit 504 then calculates an on-platedot area rate with respect to the on-plate dot image created on thebasis of the temporary exposure response characteristics.

The exposure response characteristics estimation unit 504 varies thewidth w (radius r of the circle of a cylindrical function in a case of atwo-dimensional plane) of the rectangular function to repeat calculationof the on-plate dot area rate based on the temporary exposure responsecharacteristics. The exposure response characteristics estimation unit504 then determines that temporary exposure response characteristics bywhich a measurement value of the on-plate dot area rate above becomesclosest to a calculation value of the on-plate dot area rate (anabsolute value of a difference between a measurement value of theon-plate dot area rate and a calculation value of the on-plate dot arearate becomes minimum, or equal to or less than a threshold, for example)serves as an actual exposure response characteristics.

In the example above, the exposure response characteristics are supposedas a rectangular function, but may be supposed as a gauss function, forexample. In this case, temporary exposure response characteristics maybe calculated by varying a half-value width of the gauss function. In acase of a two-dimensional plane, a curved surface to be acquired byrotating a gauss function around a normal line of a plate materialsurface (xy plane) may be supposed to calculate temporary exposureresponse characteristics by varying a half-value width of the curvedsurface.

In addition, if a beam spot diameter of a laser engraving machine isknown, a value of the beam spot diameter may serve as a width of arectangular function or a half-value width of a gauss function withoutperforming exposure response characteristics estimation.

Estimation processing of printing response characteristics will bedescribed below.

FIG. 12 is a block diagram showing a configuration of a printingresponse characteristics estimation unit 506.

In the estimation processing of printing response characteristics aswell as in the estimation processing of exposure responsecharacteristics, printing response characteristics showing a range ofpoints to be reproduced on the printed matter 400A when dots are printedon the printed matter 400A by using the printing plate 250A is supposedas a rectangular function (in fact, the printing responsecharacteristics is supposed as a function with respect to atwo-dimensional plane (a printing surface of the printed matter 400A, xyplane), however, it is supposed as above for easy explanation). Theprinting response characteristics show spread of dot portions (a blackregion in FIG. 9C) caused by spread deformation of letterpress, spreadof ink, and the like, which occurs by pressing the printing plate 250Aon a paper sheet at the time of printing. A concept of the printingresponse characteristics is similar to that of the “Point SpreadFunction” (PSF) indicating spread of a point light source in an opticsfield and the like.

Next, a calculation value of an on-printed-matter dot area ratecorresponding to an on-plate dot image is calculated by using temporaryprinting response characteristics acquired by varying a width w (radiusr of the circle of a cylindrical function in a case of a two-dimensionalplane) of a rectangular function. As the on-plate dot image, acalculated dot image in which a calculation value of an on-plate dotarea rate becomes closest to a measurement value at the time of exposureresponse characteristics estimation as well as an on-plate dot imagethat is actually measured by the plate measurement unit 500 isavailable.

As shown in FIG. 12, the printing response characteristics estimationunit 506 receives input of on-plate dot image data and temporaryprinting response characteristics to apply FFT processing to theon-plate dot image data and the temporary printing responsecharacteristics. The printing response characteristics estimation unit506 then multiplies the on-plate dot image data and the temporaryprinting response characteristics together after the FFT processing isapplied to them, and then creates an on-printed-matter dot image showingarrangement and shapes of dots to be formed on the printed matter 400Aon the basis of the temporary printing response characteristics byapplying iFFT processing to them. The printing response characteristicsestimation unit 506 then calculates an on-printed-matter dot area ratewith respect to the on-printed-matter dot image created on the basis ofthe temporary printing response characteristics.

The printing response characteristics estimation unit 506 varies thewidth w (radius r of the circle of a cylindrical function in a case of atwo-dimensional plane) of the rectangular function to repeat calculationof the on-printed-matter dot area rate based on the temporary printingresponse characteristics. The printing response characteristicsestimation unit 506 then determines that the temporary printing exposureresponse characteristics by which a measurement value of theon-printed-matter dot area rate above becomes closest to a calculationvalue of the on-printed-matter dot area rate (an absolute value of adifference between a measurement value of the on-printed-matter dot arearate and a calculation value of the on-printed-matter dot area ratebecomes minimum, or equal to or less than a threshold, for example)serves as an actual printing response characteristics.

In the example above, the printing response characteristics are supposedas a rectangular function, but may be supposed as a gauss function, forexample. In this case, temporary printing response characteristics maybe calculated by varying a half-value width of the gauss function. In acase of a two-dimensional plane, a curved surface to be acquired byrotating a gauss function around a normal line of a printing surface (xyplane) of the printed matter 400A may be supposed to calculate temporaryprinting response characteristics by varying a half-value width of thecurved surface.

Image quality evaluation value calculation processing will be describedbelow.

The image quality evaluation value calculation unit (image qualityevaluation value calculation device) 116 shown in FIG. 2 calculates animage quality evaluation value (Q) on the basis of a dot overlappingsolid brightness image created by the dot overlapping simulation unit114.

FIG. 13 is a block diagram showing calculation processing in an imagequality evaluation value calculation unit 116.

As shown in FIG. 13, the image quality evaluation value calculation unit116 applies high speed Fourier transformation (FFT) processing to a dot(tint) overlapping solid brightness image created by the dot overlappingsimulation unit 114 to acquire a power spectrum of the dot overlappingsolid brightness image. It is preferable that, in order to preventaliasing at the time of the FFT processing, the least common multiple ofthe numbers of pixels of dot threshold data corresponding to respectivecolors of CMYK serves as the number of pixels of a dot overlapping solidbrightness image. Alternatively, it is more preferable that any numberof pixels serves as a range of a dot overlapping solid brightness imageto apply a window function, in which the number of pixels within therange is indicated as 1, and the number of pixels out of the range isindicated as 0, to a pixel value of the dot overlapping solid brightnessimage, and then the FFT processing is performed.

In addition, the image quality evaluation value calculation unit 116stores visual characteristics (Visual Transfer Function: VTF) in astorage device (memory, not shown). The visual characteristics showspace frequency characteristics of human vision. As the visualcharacteristics, the VTF (Expression 2) of Dooley is available, forexample.

$\begin{matrix}{{{VTF} = {5.05\;{{\mathbb{e}}^{{- 0.138}\; u}\left( {1 - {\mathbb{e}}^{0.1\; u}} \right)}}},{u = {\frac{\pi\;{Lf}_{r}}{180}\left\lbrack {{cycles}\text{/}\deg} \right\rbrack}}} & \left\lbrack {{Expression}\mspace{14mu} 2} \right\rbrack\end{matrix}$

In Expression 2, L indicates an observation distance (mm), and f_(r)indicates a space frequency (cycles/mm).

FIG. 14 is a graph showing visual characteristics (Dooley-Shaw function)in a case where an observation distance (L) is set at 300 mm. In FIG.14, a space frequency (cycles/mm) is indicated in a horizontal axis, anda value of VTF (normalized) is indicated in a vertical axis.

A function shape of visual characteristics is not limited to theDooley-Shaw function shown in FIG. 14. A variety of characteristicsbased on a mathematical model and experimental data may be applicable,for example. In addition, since the observation distance of 300 (mm)when the visual characteristics are calculated is an example, thepresent invention is not limited to the distance. A user may determinethe observation distance L when the visual characteristics arecalculated, depending on an observation aspect of an image and the like,for example.

The image quality evaluation value calculation unit 116 applies the FFTprocessing to visual characteristics to acquire a power spectrum of thevisual characteristics. In addition, the image quality evaluation valuecalculation unit 116 calculates an image quality evaluation value on thebasis of a power spectrum acquired by multiplying a power spectrum of adot overlapping solid brightness image and the power spectrum of thevisual characteristics together. Specifically, the image qualityevaluation value calculation unit 116 calculates (A) an average value ofpower spectra, (B) a maximum value of power spectra, or (C) a weightedsum of an average value and a maximum value of power spectra, as animage quality evaluation value. The image quality evaluation valuecalculated by the image quality evaluation value calculation unit 116 isoutputted to a total evaluation value calculation unit 118.

FIGS. 15A to 15D are graphs showing examples of a power spectrum of adot overlapping solid brightness image. In FIGS. 15A to 15D, a spacefrequency (cycles/mm) is indicated in a horizontal axis, and a value ofa power spectrum (normalized) is indicated in a vertical axis.

In the power spectra of dot overlapping solid brightness imagesdescribed in FIGS. 15A to 15D, a power spectrum of visualcharacteristics (VTF) is multiplied, therefore, a power spectrum valueof a region (a frequency region in which a space frequency is about 6cycles/mm or more, for example), which is hardly recognized by humanvision, is relatively small.

In an example shown in FIG. 15A, a large peak appears in a low frequencyregion (about 1 cycle/mm) As a result, in a case of FIG. 15A, a moirecan be easily visually recognized in the printed matter 400.

In an example shown in FIG. 15B, a large peak appears in a region whosefrequency is higher than that of the low frequency region in FIG. 15Aand which can be recognized by human vision (about 4.5 cycles/mm). As aresult, in a case of FIG. 15B, a rosette pattern can be easily visuallyrecognized in the printed matter 400.

In an example shown in FIG. 15C, power spectrum values are relativelyhigh over approximately the whole region (about 0 to about 6 cycles/mm)which can be recognized by human vision, therefore, noise and granularfeeling can be easily visually recognized in the printed matter 400.

In an example shown in FIG. 15D, power spectrum values are low to theextent that deterioration in image quality of the printed matter 400 isout of the question over approximately the whole region (about 0 toabout 6 cycles/mm) which can be recognized by human vision.

Since numeric values in the vertical axis in FIGS. 15A to 15D are onlyan example, reference of a power spectrum value to which deteriorationin image quality is out of the question can vary depending on the extentof image quality to be required in the printed matter 400.

Table 2 shows calculation examples of image quality evaluation values.The examples 1 to 4 in Table 2 correspond to image quality evaluationvalues calculated on the basis of power spectra in FIGS. 15A to 15D,respectively.

TABLE 2 Image quality Example Example Example Example evaluation value 12 3 4 (A) Average value 0.12 0.12 0.26 0.10 of power spectra (B) Maximumvalue 0.89 0.94 0.61 0.38 of power spectra (C) Weighted sum of 1.01 1.060.87 0.48 an average value and a maximum value of power spectra

Even in a case where any one of (A) an average value of power spectra,(B) a maximum value of power spectra, and (C) a weighted sum of anaverage value and a maximum value of power spectra, serves as an imagequality evaluation value, as the image quality evaluation valuedecreases, the power spectrum value decreases, thereby improving imagequality. In the examples shown in Table 2, even in a case where any oneof image quality evaluation values of (A) to (C) is used, the imagequality evaluation value of the example 4 (FIG. 15D) is minimum,therefore, it is evaluated that image quality of the example 4 (FIG.15D) is most favorable.

In addition, in the examples shown in Table 2, both weight coefficientswith respect to (A) an average value of power spectra and (B) a maximumvalue of power spectra, when (C) a weighted sum of an average value anda maximum value of power spectra is calculated, are 1.0, whereby(C)=(A)+(B). A value of the weight coefficient is not limited to 1.0,therefore, a user may vary the value depending on application of theprinted matter 400, and the like. In a case, for example, where a peakof a specific frequency component, such as a moire and a rosettepattern, is to be reduced, a weight coefficient to be applied to (B) amaximum value of power spectra should be larger than a weightcoefficient to be applied to (A) an average value of power spectra. Inaddition, in a case where noise and granular feeling, in which there isno peak in a specific frequency component, are to be reduced, a weightcoefficient to be applied to (A) an average value of power spectrashould be larger than a weight coefficient to be applied to (B) amaximum value of power spectra.

Total evaluation value calculation processing will be described below.

The total evaluation value calculation unit (total evaluation valuecalculation device) 118 calculates a total evaluation value (V) for eachof candidates of device signal values on the basis of a color evaluationvalue (E) for each of the candidates of device signal values, the colorevaluation value (E) being received through the color evaluation valueholding unit 110, and an image quality evaluation value (Q) for each ofthe candidates of device signal values, the image quality evaluationvalue (Q) being calculated by the image quality evaluation valuecalculation unit 116. The total evaluation value (V) is calculated byweighting addition shown in Expression 3 below in a case where a weightcoefficient to be applied to the color evaluation value is indicated asWe, and a weight coefficient to be applied to the image qualityevaluation value is indicated as Wq.V=We×E+wq×Q  [Expression 3]

In addition, the total evaluation value may be calculated by adding anevaluation value other than the color evaluation value (E) and the imagequality evaluation value (Q). Examples of an evaluation value other thanthe color evaluation value (E) and the image quality evaluation value(Q) are as follows: an ink cost evaluation value; an evaluation valuerelated to an expiration date of ink (an index for deteiminingdistribution of ink so that ink close to an expiration date is consumedon a priority basis); an evaluation value of spectral characteristics(whether to be close to spectral characteristics of a spot color ink ofa reproduction target), and the like.

The ink cost evaluation value (I) for evaluating ink cost is calculatedby an ink cost evaluation value calculation device (not shown) asdescribed below. The ink cost evaluation value I can be expressed by thefollowing expression, where a dot area rate of each of CMYK candidatesis indicated as C, M, Y, and K, and cost of each ink of CMYK per 1 kg isindicated as follows: C ink is Ic, M ink is Im, Y ink is Iy, and K inkis Ik.I=Ic×C+Im×M+Iy×Y+Ik×K  [Expression 4]

A total evaluation value is expressed by Expression 5 described below,where a weight coefficient to be applied to the ink cost evaluationvalue is indicated as Wi.V=We×E+Wq×Q+Wi×I  [Expression 5]

In Expression 3 or 5 above, a user may determine values of the weightcoefficients We, Wq, and Wi through an input device (weight coefficientsetting device) depending on application of the printed matter 400 andthe like. In a case, for example, where a higher premium is put onreproducibility of colors than deterioration in image quality caused bya moire, a rosette pattern, or noise (in a case of an image containingcolors with positions close to each other in a hue circle, for example),We should be larger than Wq. In addition, in a case where deteriorationin image quality and increase in color difference should be preventedeven if ink cost increases, a value of Wi should be lower than values ofWe and Wq or Expression 3 should be used.

Optimum device signal value determination processing will be describedbelow.

The optimal values determination unit 120 determines optimum devicesignal values (CMYK) from among candidates of device signal values onthe basis of the total evaluation value (V) calculated by the totalevaluation value calculation unit 118. In addition, the optimal valuesdetermination unit 120 determines CMYK where the total evaluation value(V) becomes minimum as an optimum device signal value (color separationresult).

A color separation method will be described below.

FIG. 16 is a flow chart showing color separation method according to oneembodiment of the present invention.

First, the image processing apparatus 100 specifies target values (spotcolors) of colors to be reproduced on the printed matter 400 (step S10).Specifying spot colors is performed by receiving input of spot color IDsor by allowing the colorimetry unit 104 to measure color of color chipsand the like.

The device signal value candidate calculation unit 106 then acquires aprint profile from the print profile storage unit 108. In addition, thedevice signal value candidate calculation unit 106 calculates acandidate of device signal values (device signal value candidate), whichcan reproduce a target Lab value to be reproduced on recording paper(target Lab value) within a range of allowable color differences, on thebasis of the acquired print profile (step S12). A color differenceacquired for each of candidates of device signal values in the step S12is held in the color evaluation value holding unit 110 as a colorevaluation value of each of device signal value candidates (step S14).

Next, the dot overlapping simulation unit 114 simulates an image to beprinted when a dot image corresponding to each of color materials(process colors, for example) is printed (step S16). The image qualityevaluation value calculation unit 116 calculates an image qualityevaluation value of each of device signal value candidates on the basisof the simulation result (step S18).

Next, the total evaluation value calculation unit 118 calculates a totalevaluation value for each device signal value candidate on the basis ofthe color evaluation value and the image quality evaluation value above(step S20), and an optimum device signal value is determined from amongthe device signal value candidates on the basis of the total evaluationvalue (step S22). In the step S20, the total evaluation value may becalculated by using an evaluation value other than the color evaluationvalue and the image quality evaluation value. In addition, in the stepS20, a user may input a weight coefficient to be applied to anevaluation value to be used in calculation of the total evaluationvalue.

According to the present embodiment, since color separation is performedby using an image quality evaluation value calculated by using a powerspectrum of a dot overlapping solid brightness image and a powerspectrum of visual characteristics in addition to a color differenceevaluation value showing an amplitude of color difference, it ispossible to prevent an image formation such as a moire and a rosettepattern from occurring while color difference occurring between a spotcolor and a color system is prevented. In addition, according to thepresent embodiment, by performing dot overlapping simulation, it ispossible to achieve optimum color separation capable of preventing acolor difference and preventing an image formation from occurring by asimpler calculation without printing and measuring a patch for eachdevice signal value.

Others will be described below.

In the present embodiment, an optimum device signal value is to bedetermined corresponding to designation by a spot color ID or a spotcolor measured by the colorimetry unit 104, however, the presentinvention is not limited to the manner above. It is allowed, forexample, to calculate an optimum color separation result for allavailable spot colors (spot colors listed in a color sample book of anink manufacturer, such as Pantone, for example) in advance to make adatabase (tabulation). In this case, the image processing apparatus 100may acquire optimum color separation by referring to a database (table)on the basis of a spot color designated by a spot color ID or measuredby the colorimetry unit 104.

In addition, a user may select an optimum device signal value inaccordance with an evaluation result (evaluation value) of a colorseparation result.

FIG. 17 is a diagram showing a Graphical User Interface (GUI) to selecta color separation result.

In examples shown in FIG. 17, there are displayed color differenceevaluation values with respect to three sets of device signal values(CMYK values of (c1, m1, y1, k1), (c2, m2, y2, k2), and (c3, m3, y3,k3)), image quality evaluation values and ink cost evaluation values, aswell as target colors of an object to be printed on the printed matter400 and samples of reproduction colors to be actually printed. Althoughthe color difference evaluation values, the image quality evaluationvalues, and ink cost evaluation values are displayed at four levels ofA, B, C, and D for easy understanding by a user (a color difference of 0to less than 1.5, 1.5 to less than 3.0, 3.0 to less than 5.0, and 5.0 ormore, are indicated as A, B, C, and D, respectively, for example),numeric values of respective evaluation values may be displayed.

A user may operate a “determination” button displayed on a right side ofeach of the CMYK values (c1, m1, y1, k1), (c2, m2, y2, k2), and (c3, m3,y3, k3) by using a pointing device (not shown) and the like so that adesired CMYK value (color separation result) can be selected.

In the GUI shown in FIG. 17, for example, a dot overlapping solidbrightness image acquired by the dot overlapping simulation may bedisplayed. In addition, a GUI for calculating total evaluation valuesformed by specifying a weight coefficient to be applied to each of theevaluation values may be displayed together.

A color separation method in accordance with the present invention canbe provided also as computer-readable program codes to allow a processorto perform the processing described above, non-transitory andcomputer-readable recording media storing the program codes (an opticaldisk such as a Compact Disc (CD), a Digital Versatile Disc (DVD), and aBlu-ray (registered trademark) Disc (BD), a magnetism disk such as ahard disk, and a magneto-optical disk, and a Universal Serial Bus (USB)memory, for example), and a computer program product storing executablecodes for the method described above.

What is claimed is:
 1. A color separation apparatus comprising: a targetvalue acquisition device which acquires target values of colors to bereproduced in a printer from continuous-tone image data when the printercreates binary image data showing shape and arrangement of dotsconstituting an image for each of color materials; a dot threshold dataacquisition device which acquires dot threshold data includinginformation on a threshold for each of the dots for converting thecontinuous-tone image data into binary image data for each of the colormaterials; a print profile acquisition device which acquires a printprofile showing correspondence between a device signal value and a valueof a color system in the printer; and a color separation device whichallows the printer to calculate candidates of the device signal value onthe basis of the target values of colors acquired by the target valueacquisition device and the print profile, and determines a device signalvalue for reproducing colors corresponding to the target values fromamong the candidates of the device signal value on the basis of the dotthreshold data and the print profile.
 2. The color separation apparatusaccording to claim 1, further comprising: a color evaluation valueholding device which holds a color evaluation value showing a colordifference between a device signal value and a color corresponding tothe target value for each of the candidates of the device signal values;and an image quality evaluation value calculation device whichcalculates an image quality evaluation value for each of the candidatesof the device signal values on the basis of the dot threshold data andthe print profile, wherein the color separation device determines devicesignal values for reproducing colors corresponding to the target valueson the basis of the color evaluation value and the image qualityevaluation value.
 3. The color separation apparatus according to claim2, further comprising: a simulation device which applies binary codedprocessing to the candidates of the device signal values by using thedot threshold data to create binary image data for each of colormaterials for each of the candidates, and simulates an image formed bythe printer on a printed matter by superimposing binary image data foreach of the color materials, wherein the image quality evaluation valuecalculation device calculates the image quality evaluation value foreach of the candidates on the basis of a result of the simulation of theimage to be formed on the printed matter.
 4. The color separationapparatus according to claim 3, wherein the simulation device createsthe binary image data in consideration of change in a shape of the dotoccurring when the dot is printed by the printer on the basis ofresponse characteristics at the time of forming dots corresponding tothe binary image data in the printer.
 5. The color separation apparatusaccording to claim 2, further comprising: an ink cost evaluation valuecalculation device which calculates an ink cost evaluation value foreach of the candidates of the device signal values on the basis of a dotarea rate of each of color materials, wherein the color separationdevice determines the device signal values for reproducing colorscorresponding to the target values on the basis of the color evaluationvalue, the image quality evaluation value, and the ink cost evaluationvalue.
 6. The color separation apparatus according to claim 3, furthercomprising: an ink cost evaluation value calculation device whichcalculates an ink cost evaluation value for each of the candidates ofthe device signal values on the basis of a dot area rate of each ofcolor materials, wherein the color separation device determines thedevice signal values for reproducing colors corresponding to the targetvalues on the basis of the color evaluation value, the image qualityevaluation value, and the ink cost evaluation value.
 7. The colorseparation apparatus according to claim 4, further comprising: an inkcost evaluation value calculation device which calculates an ink costevaluation value for each of the candidates of the device signal valueson the basis of a dot area rate of each of color materials, wherein thecolor separation device determines the device signal values forreproducing colors corresponding to the target values on the basis ofthe color evaluation value, the image quality evaluation value, and theink cost evaluation value.
 8. The color separation apparatus accordingto claim 2, further comprising: a weight coefficient setting devicewhich determines a weight coefficient for each of the evaluation values;and a total evaluation value calculation device which calculates a totalevaluation value for each of the candidates of the device signal valuesby performing weighting addition of each of the evaluation values byusing the weight coefficient determined by the weight coefficientsetting device, wherein the color separation device determines devicesignal values for reproducing colors corresponding to the target valueson the basis of the total evaluation value.
 9. The color separationapparatus according to claim 3, further comprising: a weight coefficientsetting device which determines a weight coefficient for each of theevaluation values; and a total evaluation value calculation device whichcalculates a total evaluation value for each of the candidates of thedevice signal values by performing weighting addition of each of theevaluation values by using the weight coefficient determined by theweight coefficient setting device, wherein the color separation devicedetermines device signal values for reproducing colors corresponding tothe target values on the basis of the total evaluation value.
 10. Thecolor separation apparatus according to claim 4, further comprising: aweight coefficient setting device which determines a weight coefficientfor each of the evaluation values; and a total evaluation valuecalculation device which calculates a total evaluation value for each ofthe candidates of the device signal values by performing weightingaddition of each of the evaluation values by using the weightcoefficient determined by the weight coefficient setting device, whereinthe color separation device determines device signal values forreproducing colors corresponding to the target values on the basis ofthe total evaluation value.
 11. The color separation apparatus accordingto claim 5, further comprising: a weight coefficient setting devicewhich determines a weight coefficient for each of the evaluation values;and a total evaluation value calculation device which calculates a totalevaluation value for each of the candidates of the device signal valuesby performing weighting addition of each of the evaluation values byusing the weight coefficient determined by the weight coefficientsetting device, wherein the color separation device determines devicesignal values for reproducing colors corresponding to the target valueson the basis of the total evaluation value.
 12. The color separationapparatus according to claim 6, further comprising: a weight coefficientsetting device which determines a weight coefficient for each of theevaluation values; and a total evaluation value calculation device whichcalculates a total evaluation value for each of the candidates of thedevice signal values by performing weighting addition of each of theevaluation values by using the weight coefficient determined by theweight coefficient setting device, wherein the color separation devicedetermines device signal values for reproducing colors corresponding tothe target values on the basis of the total evaluation value.
 13. Thecolor separation apparatus according to claim 7, further comprising: aweight coefficient setting device which determines a weight coefficientfor each of the evaluation values; and a total evaluation valuecalculation device which calculates a total evaluation value for each ofthe candidates of the device signal values by performing weightingaddition of each of the evaluation values by using the weightcoefficient determined by the weight coefficient setting device, whereinthe color separation device determines device signal values forreproducing colors corresponding to the target values on the basis ofthe total evaluation value.
 14. The color separation apparatus accordingto claim 1, wherein the target value acquisition device acquires targetvalues of colors to be reproduced in a printer by receiving input of anidentifier for identifying a color or by measuring the colors.
 15. Thecolor separation apparatus according to claim 2, wherein the targetvalue acquisition device acquires target values of colors to bereproduced in a printer by receiving input of an identifier foridentifying a color or by measuring the colors.
 16. The color separationapparatus according to claim 3, wherein the target value acquisitiondevice acquires target values of colors to be reproduced in a printer byreceiving input of an identifier for identifying a color or by measuringthe colors.
 17. The color separation apparatus according to claim 4,wherein the target value acquisition device acquires target values ofcolors to be reproduced in a printer by receiving input of an identifierfor identifying a color or by measuring the colors.
 18. The colorseparation apparatus according to claim 5, wherein the target valueacquisition device acquires target values of colors to be reproduced ina printer by receiving input of an identifier for identifying a color orby measuring the colors.
 19. A color separation method performed by acolor separation apparatus, comprising the steps of: acquiring targetvalues of colors to be reproduced in a printer from continuous-toneimage data when the printer creates binary image data showing shape andarrangement of dots constituting an image for each of color materials;acquiring dot threshold data including information on a threshold foreach of the dots for converting the continuous-tone image data intobinary image data for each of the color materials; acquiring a printprofile showing correspondence between a device signal value and a valueof a color system in the printer; and a color separation step of:allowing the printer to calculate candidates of the device signal valueon the basis of the target values of colors acquired in the target valueacquisition step and the print profile, and determining a device signalvalue for reproducing colors corresponding to the target values fromamong the candidates of the device signal value on the basis of the dotthreshold data and the print profile.
 20. A non-transitorycomputer-readable medium storing a color separation program allowing acomputer to realize the functions of: a target value acquisition ofacquiring target values of colors to be reproduced in a printer fromcontinuous-tone image data when the printer creates binary image datashowing shape and arrangement of dots constituting an image for each ofcolor materials; a dot threshold data acquisition of acquiring dotthreshold data including information on a threshold for each of the dotsfor converting the continuous-tone image data into binary image data foreach of the color materials; a print profile acquisition of acquiring aprint profile showing correspondence between a device signal value and avalue of a color system in the printer; and a color separation ofallowing the printer to calculate candidates of the device signal valueon the basis of the target values of colors acquired by the target valueacquisition function and the print profile, and determining a devicesignal value for reproducing colors corresponding to the target valuesfrom among the candidates of the device signal value on the basis of thedot threshold data and the print profile.